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hehehe...that's always easier. Or nmap/saint/satan if your a linux user...just thought I'd say. Netstat for me, is getting old, I like it when people reinvent the wheel. As has been done, in revolutions since long before even written glyphs.

"Beware the Jabberwock, my son!The jaws that bite, the claws that catch!Beware the Jubjub bird, and shunThe frumious Bandersnatch!"

Heh, now study the TCP/IP protocols on the ports you have questions about. No port is reserved for a trojan. A trojan may use a specific port but when the brain trust developed TCP/IP protocol suit they did not say lets reserve port XXXX for a trojan

Think of TCP and UDP as similar but different protocols. Also study the OSI modle. Here is a basic TCP/IP protocol stack in relation to the OSI model

As you can see both UDP and TCP are on the transport layer of the OSI modle. This is because they are both used for transport. Read and understand what I just gave you and you will have better questions to ask.

If you want to learn to network security.. Study the TCP/IP prtocol stack. Everything you do in a network involves at least several protocols. Learn how the work. How networking in general works. What each layer of the OSI modle describes. How the TCP/IP modle stack is laid out *When authentication starts* Oh and here is where number systems come in handy...

I made perfect sense. Any program can open a port, the ports aren't assigned to anything, a program can run on any port it wants; just some ports have more common services running on it than others...

21 is known for FTP because most FTP clients run on it, hence 21 is most commonly used for FTP, however if you don't have an FTP server you can run a program on that same port and accept connections. With FTP servers also you can tell it to listen on any other port, even 25 which is for SMTP...